Microbolometer and its manufacturing method

ABSTRACT

The invention concerns a microbolometer comprising a suspended part containing radiation-sensitive elements and consisting of a set of first zones and a set of second zones, the two sets being superimposed; furthermore, the materials constituting said zones and have thermal expansion coefficients sufficiently different for said suspended part to be deformed under the effect of a rise in temperature to be urged into contact with the substrate when the contact zone reaches a temperature T c  less than the destruction temperature T d  of the microbolometer. The invention is applicable to radiation detectors comprising an assembly of such microbolometers, and to various appliances comprising at least such a radiation detector.

PRIORITY CLAIM

This application claims priority to French Patent Application No.0108552, filed Jun. 28, 2001

TECHNICAL FIELD

The invention relates to the manufacture of microbolometers, andradiation detectors comprising an array of microbolometers.

BACKGROUND

A bolometer is a device designed to measure the intensity of radiation,usually situated in the infrared, to which it is subjected, bytransforming the energy of this radiation into thermal energy. Theresultant heating of the bolometer causes the variation of an electricvariable such as the electrical resistance of a conductor connected to acircuit outside the bolometer. In the case for example of a detectorcomprising a microbolometer matrix, this electric circuit, referred toas “readout” circuit, performs the functions of matrix addressing andreading stimuli sent to each microbolometer, and converts the resultantsignals to a format that can be used in particular for imaging (forexample in the form of a video signal). In order to obtain the bestpossible performances, the microbolometers are operated under arelatively low gas pressure (or under moderate pressure of a gas withlow thermal conductivity), in order for the thermal dissipation due tothis gas to be negligible vis-à-vis the intrinsic thermal conductance ofthe microbolometers.

The readout circuit measures the relative variation of said electricvariable (attached to the bolometer's sensitive element) which dependson the temperature. In the case of non-cooled detectors (which aresimpler and less expensive than detectors equipped with a coolingsystem), the bolometer's temperature variation is for its partproportional to the power of radiation received, the proportionalityconstant (called “thermal resistance” and which we shall designateR_(thb)) usually being between 5·10⁶ and 2˜10⁷ K/W.

FIG. 1 relates to a standard microbolometer with a useful surface areaof 40 μm×30 μm, having a thermal resistance R_(thb) equal to 10⁷ K/W andunit absorption, placed at the focus of an optical system having anaperture angle of 53° equipped with a spectral filter offering aconstant transmission equal to 0.9 in the infrared (more precisely,within a range of wavelength between 8 and 14 μm). The curves representthe radiation power received and the increase in temperature of themicrobolometer as a function of the temperature of the radiation source,regarded as a “black body”. This heating takes place even if the imagingsystem is not in operation (i.e. in the absence of electrical stimuli).

For a microbolometer provided with a “perfect” spectral filter, i.e.having zero transmission for all the wavelengths outside theabovementioned range, the increase in the microbolometer's equilibriumtemperature with the temperature of the source (solid curve) is linearonce the latter exceeds 2000 K. For example, the static observation of asource such as the sun (approximately 6000 K) causes an increase of theorder of 100 K in the temperature of the microbolometer.

In practice, the spectral filters, even those of good quality, installedin infrared imaging systems are not perfect, in that they allow thepassage of a low, but not zero, power of radiation at wavelengthssituated outside the filter's theoretical operating range. Very hotsources however emit much more power in the visible range than in theinfrared. Therefore, the optical power received on the detector, outsidethe filter's infrared theoretical range, can be considerable, and even,on occasion, preponderant with a very high source temperature and/or forfilters of mediocre quality. In this case, the heating estimateindicated previously can be considerably lower than the actual value.The dashed curve in FIG. 1 shows the effective variation in thetemperature of the microbolometer when it is provided with a“non-perfect” filter, having a transmission equal to 10⁻³ outside theabovementioned infrared range. Of course, the use of higher thermalresistances, advantageous under normal conditions of use since theyincrease the microbolometer's sensitivity, means even greater heating.

Microbolometers are usually designed to operate, and this is one oftheir advantages, close to the ambient temperature. But thesemicrobolometers are constituted by materials (such as vanadium oxides oramorphous silicon) which exhibit a permanent, or at least durable changein their electrical characteristics (and also possibly mechanicaldeformation of their structure) for such rises in temperature, eventemporary. Very high illuminations can even lead to their physicaldestruction.

Moreover, even if temporarily over-illuminated microbolometers are notdestroyed, a change, even temporary, in the electrical resistance valuesof the image points (probable or inevitable beyond 100 to 200 K heating,for most of the microbolometers) renders inoperative the “offset”compensation electrical device, usually integrated in the readoutcircuit, which has been calibrated for the original spatial distributionof the individual resistances on the microbolometer.

Consequently, the observation, even transient (over a period of theorder of the microbolometer's thermal time constant, i.e. usually fromsome milliseconds to some tens of milliseconds) of very hot sources, forexample the filament of an incandescent lamp (approximately 3000 K), orthe sun, is usually fatal for this type of device.

As imaging systems based on detectors of the bolometric type accordingto the state of the art are not compatible with the observation, eventemporary or accidental, of very intense sources, it is necessary tolimit the use of these systems to environments that are essentially notvery aggressive, or to take constraining precautions during the use ofsuch systems.

SUMMARY

The object of the invention is to propose microbolometers that are lessvulnerable to over-illuminations than are the standard microbolometers,thanks to an extension of the range of admissible radiation intensities,without however reducing the device's sensitivity to moderate radiationintensities.

To this end, the invention proposes a method for manufacturingmicrobolometers on a substrate, said method being remarkable in that itcomprises in particular, for each microbolometer, the following steps:

-   -   the deposition and etching of several layers, including a set of        layers intended to constitute a suspended part of the        microbolometer containing a radiation-sensitive element, and    -   the formation in said suspended part of a set of first zones and        a set of second zones, the two sets being superimposed,        and in that the materials constituting said first zones possess        thermal expansion coefficients sufficiently different from the        materials constituting said second zones for the suspended part        to be deformed under the effect of a rise in temperature to the        point of coming into contact with the substrate when the contact        zone reaches a temperature T_(c) below the destruction        temperature T_(d) of the microbolometer.

By means of these arrangements, from the moment when the suspended partof the microbolometer, during use, reaches said temperature T_(c), thispart cools down due to its contact with the substrate. Themicrobolometers according to the invention can consequently support aconsiderably extended range of scene temperatures compared with themicrobolometers according to the prior art.

It will be noted that the microbolometers according to the invention canbe entirely manufactured using microelectronics techniques, and incollective manner.

Correlatively, the invention relates to a microbolometer comprising asuspended part containing a radiation-sensitive element, saidmicrobolometer being remarkable in that said suspended part isconstituted by a set of first zones and a set of second zones, the twosets being superimposed, and in that the materials constituting saidfirst zones possess thermal expansion coefficients sufficientlydifferent from the materials constituting said second zones for thesuspended part to be deformed under the effect of a rise in temperatureto the point of coming into contact with the substrate when the contactzone reaches a temperature T_(c) below the destruction temperature T_(d)of the microbolometer.

Thus, the microbolometers according to the invention comprise, by virtueof their very constitution, suitable elements for protecting themagainst high scene temperatures.

Said first zones can, according to the particular needs of a personskilled in the art, be situated either “below” or “above” said secondzones. Given that the first zones can, optionally, possess a thermalexpansion coefficient smaller or greater than that of the second zones,the suspended part of the microbolometer according to the invention willbe deformed under the effect of a rise in temperature into a shape whichcan be convex or concave.

According to preferred characteristics, the microbolometer according tothe invention will have protuberances emerging from the suspended partand turned towards the substrate and/or protuberances emerging from thesubstrate and turned towards the suspended part.

By means of these arrangements, when the suspended part comes, accordingto the invention, into contact with the substrate, the zones ofeffective contact get to be limited to the top of said protuberances.This prevents the suspended part adhering too strongly to the substrate,and therefore facilitates the return of the suspended part to itsnominal position when the microbolometer cools down.

According to other preferred characteristics, the suspended part of themicrobolometer includes support arms resting locally on the substrate bymeans of struts.

By means of these arrangements, the suspended part is firmly held, evenwhen it is deformed under the effect of a rise in temperature, by thesesupport arms, which for their part will expand relatively little, as oneof their ends is kept at the temperature of the substrate by means ofsaid struts.

Finally it will be noted that, depending on the choices of materials andstructure made by a person skilled in the art, the zone of the suspendedpart of the microbolometer intended to come into contact with thesubstrate at the temperature T_(c) can comprise the centre of thesuspended part, or can be remote from this centre.

The invention also relates to radiation detectors each comprising anarray of microbolometers as described briefly above, and variousobservation or measurement devices incorporating at least one suchradiation detector. These devices can for example be imaging systemsoperating in the infrared.

BRIEF DESCRIPTION OF THE DRAWING

Other aspects and advantages of the invention will become apparent onreading the detailed description to be found below, of particularembodiments given as non-limiting examples. This description refers tothe annexed drawings, in which:

FIG. 1 represents the temperature reached by a standard microbolometeras a function of the black-body temperature of the of the source;

FIG. 2 a is a top view of the assembly obtained after a first step inthe manufacture of microbolometers according to a first embodiment ofthe invention;

FIG. 2 b is a sectional view along the line AA of the assemblyillustrated in FIG. 2 a;

FIG. 3 a is a top view of the assembly obtained after a second step inthe manufacture of microbolometers according to the first embodiment ofthe invention;

FIG. 3 b is a sectional view along the line AA of the assemblyillustrated in FIG. 3 a;

FIG. 4 a is a top view of the assembly obtained after a third step inthe manufacture of microbolometers according to the first embodiment ofthe invention;

FIG. 4 b is a sectional view along the line AA of the assemblyillustrated in FIG. 4 a;

FIG. 5 a is a top view of the assembly obtained after a fourth step inthe manufacture of microbolometers according to the first embodiment ofthe invention;

FIG. 5 b is a sectional view along the line AA of the assemblyillustrated in FIG. 5 a;

FIG. 6 a is a top view of the assembly obtained after a fifth and laststep in the manufacture of microbolometers according to the firstembodiment of the invention;

FIG. 6 b is a sectional view along the line AA of the assemblyillustrated in FIG. 6 a;

FIGS. 7 a and 7 b illustrate the operation of a microbolometerconstructed according to the first embodiment of the invention, FIG. 7 abeing a top view and FIG. 7 b a sectional view along the line AA of FIG.7 a;

FIGS. 8 a and 8 b illustrate the operation of a microbolometerconstructed according to a second embodiment of the invention, FIG. 8 abeing a top view and FIG. 8 b a sectional view along the line AA of FIG.8 a; and

FIG. 9 is a graph of the temperature reached by a microbolometeraccording to the invention as a function of the power of the radiationreceived.

DETAILED DESCRIPTION

To begin with, a description will be given of the successive steps of amethod for manufacturing microbolometers according to a first embodimentof the invention.

FIGS. 2 a and 2 b respectively show a top view and a local section ofthe assembly obtained after a first step of this method.

Initially, in standard manner, there are placed in a substrate 1(usually of silicon, and a few μm thick), electric circuits (notrepresented), capable of providing, on the one hand, the reading stimulifor the microbolometers, and on the other hand the processing of thesignal resulting from the illumination of these microbolometers via asuitable optical system.

The microbolometers are usually arranged in order to form a matrix or astrip. The usual dimension of an elementary microbolometer in eachdirection parallel to the substrate 1 is usually of the order of 20 to50 μm.

The standard methods for manufacturing detectors of this type compriseinitial steps carried out directly on the surface of an electriccircuit, in “monolithic” (i.e. in a continuous sequence of operations onthe same substrate) or “hybrid” mode (with application to a substrate ofprefabricated elements). These steps involve usual techniques of themicroelectronics industry, in particular mass-production techniques,usually involving some tens to some hundreds of detectors arranged onthe same substrate (“wafer level”). During these initial steps, theactual bolometric units ensuring the optical absorption functions andhaving a resistance that can vary with the temperature are placed on thesurface of a “sacrificial” layer, which means that this layer (usuallymade of polyimide, polycrystalline silicon, or of metal such as copperor aluminium) is eliminated at the end of the process (by combustion inan oxygen plasma for example in the case of polyimide), in order toleave the microbolometer structures suspended above the substrate.

By means of this production method, the space between two adjacentmicrobolometers can be kept to a minimum, in order to allow theirphysical and electrical separation with a maximum fill factor.

It will be assumed in order to simplify the present description that nomicrobolometer has any part in common with the adjacent microbolometers.In other cases, a person skilled in the art will be able to adapt themanufacturing steps, making use of his ordinary abilities.

In accordance with the established art, there is deposited on thesubstrate 1 a metal layer 6, for example of aluminium, which is definedby subsequent etching, for example, the contour indicated in FIG. 2 a(dotted line) by means of a first lithographic mask. This layer 6comprises one or more parts 6′, the role of which is to reflect theinfrared radiation crossing the microbolometer, in order to improve itsabsorption yield, and several parts 6″ serving as points of electricalcontact with the readout circuit accommodated in the substrate 1.

Then a sacrificial layer 7 is deposited, for example of polyimide, whichis annealed at a temperature sufficient for it to be able to support theremaining operations. Preferably, this layer 7 will be approximately 2.5μm thick, in order to optimize the bolometer's optical yield (by theconstitution of a “quarter-wave” cavity after elimination of thissacrificial layer) for wavelengths included within the infrared region.

According to this first embodiment of the invention, there is thendeposited on this layer 7 a layer 8 of material sensitive to variationsin temperature, for example 100 nanometers of amorphous silicon, ofadequate resistivity, then a layer 9 of an electrical insulatingmaterial such as silicon oxide, from 5 to 20 nanometers thick forexample.

This layer 9 is then etched opposite the zone 6′ using a secondlithographic mask, along the contour illustrated by a solid line in FIG.2 a. Then a layer 10 is deposited, which simultaneously covers theetched layer 9 and the layer 8 beyond the etched layer 9. This layer 10,which is made of a metal material such as titanium nitride TiN, willprovide the electrodes of the sensitive part of the microbolometer, andalso serve as infrared absorber. In order to optimize themicrobolometer's yield, it will preferably have a layer resistance ofapproximately 400Ω, which usually corresponds to a thickness of lessthan 10 nanometers.

In a second step of this embodiment, illustrated in FIGS. 3 a and 3 b,using a third lithographic mask, by an appropriate dry etching process,the layers 10, 9 and 8 are etched, then part of the layer 7. Then, onthe layer 10 and in the etched zones, a metal layer 11 is deposited,preferably of aluminium and between 50 to 200 nanometers thick.

This etching defines at least one first aperture facing a zone 6′, ofrelatively small dimensions compared with the dimensions of themicrobolometer. As can be seen in FIG. 3 b, the depth of etching in thelayer 7 defines the height of protuberances 4 from the layer 11associated with these first apertures. This height will preferably bebetween 0.5 μm and 1.5 μm, in order to leave a sufficient margin betweenthe end of the protuberances 4 and the surface of the layer 6.

During a third step, represented in FIGS. 4 a and 4 b, using a fourthlithographic mask, second apertures are defined, situated opposite thezones 6″, which will make it possible, using an appropriate dry etchingprocess, to etch the layers 11, 10, 8 and 7 over their whole thicknessthrough to the surface of the layers 6″.

The choice of aluminium for the layer 11 makes it possible to use thelatter as a mineral counter-mask for the etching of this sequence ofmaterials.

Then at least one essentially metal layer is deposited, in order toproduce parts 3 associated with said second apertures. These parts 3 aredelimited at the surface by etching using a fifth lithographic maskrepresented in FIG. 4 a. Struts supporting the mechanical structure areproduced, as well as the electrical connections between the readoutcircuit and the future electrodes (see FIGS. 5 a and 5 b) of themicrobolometer.

In this embodiment, a design with four struts 3 has been chosen, butother choices are possible, bearing in mind the need to producerelatively rigid structures as explained below.

During a fourth step, a sixth lithographic mask is used, in order todefine, by chemical etching, patterns in the layer 11, as represented inFIGS. 5 a and 5 b. These patterns comprise at least one surface 2B,essentially oblong in design, the direction of which is indicated by theY axis in the figures, and the ends of which are situated at theprotuberances 4, which must thus be protected, and also if necessary thezones 3, during this chemical etching.

The set of layers 8, 9 and 10 (essentially the layer 8 in thisembodiment) defines a part 2A which, according to the invention, has athermal expansion coefficient appreciably different from that of thepart 2B, the parts 2A and 2B being superimposed and together forming thesuspended part 2 of the microbolometer. If, for example, aluminium isused for the parts 2B, these will possess a thermal expansioncoefficient equal to 25×10⁻⁴/K whereas the silicon of which the layer 8is made possesses a thermal expansion coefficient equal to 5×10⁻⁴/k. Avariant that can be chosen is to make the parts 2B from other materialshaving the same properties, such as silicon nitride, titanium nitride,tungsten or tungsten silicide; by doing so, preference will be given tomaterials that have a high modulus of elasticity compared with thematerials constituting the parts 2A, in order to promote the deformationof the overall structure under the effect of heat, as described belowwith reference to FIGS. 7 a and 7 b.

Using a seventh lithographic mask, the metal layer 10 is then etchedlocally, in order to separate it into parts forming electrodes 10A,situated in particular at the zones 3, and into parts forming aninfrared absorber 10B, as shown in FIGS. 5 a and 5 b. It will be notedthat in this embodiment, the parts 2B are in electrical contact, on theone hand, with the parts 10B over their whole surface, and on the otherhand with the sensitive element 8 of the detector by means of theprotuberances 4. This is why, in this embodiment, the conductor 10B mustbe separated into electrically isolated zones each containing a singlepart 2B, in order to avoid short-circuiting this sensitive element(electrical resistance) 8. FIG. 5 a shows (in solid lines) the zones ofseparation between the parts 10A and 10B as well as between the parts10B themselves, all these zones of separation being here orientedparallel to the Y axis.

Other contours could have been used for the various polygons defined bythe seventh mask, as well as for the polygons connected to the readoutcircuit (such as 10A), the polygon or polygons with floating potential(such as 10B), on condition, of course, that the points short-circuitedby the zone or zones 2B are equipotential. In the opposite case, aperson skilled in the art will be able to take the necessary precautionsduring assembly, by inserting for example a dielectric insulation layerin the right place, or even by using an insulating material for theparts 2B.

Finally, during a fifth step, an eighth and last lithographic mask isused in order to define the final contour of the microbolometers by dryetching of the layers 10A and 8 (as well as, partially, of the layer 7),as represented in FIGS. 6 a and 6 b. This contour retains the central,optically active, part 10B of the microbolometer, and defines thermalinsulation arms 12 (numbering four in this embodiment), which have asignificant effect on the value acquired by the microbolometer's “own”thermal resistance R_(thb), i.e. of its suspended part. In this firstembodiment, these arms 12 extend parallel to the Y axis, left and right,and on either side, of the median part of the suspended part 2.

This sequence of manufacturing steps ends with the elimination of thesacrificial layer 7 by dry oxidation, using well known techniques.

Thus, using this method according to the invention, a microbolometer hasbeen obtained comprising a suspended part 2 containing aradiation-sensitive element and constituted by a set of first zones 2Aand a set of second zones 2B, the two sets being superimposed, and thematerials constituting the first zones 2A possessing appreciablydifferent thermal expansion coefficients.

The operation of a microbolometer according to this first embodiment ofthe invention is illustrated in FIGS. 7 a and 7 b.

The temperature of the substrate T_(sub) is generally monitored by anappropriate system, at a moderate temperature (ordinarily, slightlyabove the ambient temperature) for reasons of temporal stability of theelectric operating points of the readout circuit and of themicrobolometers (during non-operation, the temperature T_(sub) is simplyequal to the ambient temperature). The temperature of themicrobolometers depends on the temperature of the source observed, aswas mentioned in the introduction, as well as on the readout mode (i.e.the way in which the temperature readout stimuli are applied, via theelectric circuit). Generally, the average temperature T_(bol) of themicrobolometers will be higher than that of the substrate, thedifference ranging from some degrees to some tens of degrees.

In the case of excessive heating, for example due to a transient ordurable illumination, the arms 12 will expand relatively little, due tothe choice of materials from which they are made, and also because oneof their ends is kept at the temperature of the substrate via the struts3. In a first approximation, it can be considered that the set ofstructures comprising the struts 3, the support arms 12, and theconnecting zones left and right of the central zone of themicrobolometer in FIG. 7 a, does not undergo any deformation. The sameapplies, by symmetry, to the zone situated along the median (dashed)line parallel to the X axis: all these zones remain at approximately thesame distance from the substrate in the absence of mechanical contactwith the latter. On the other hand, the zones situated on either side ofthis dashed line curve downwards due to the differential thermalexpansion between the parts 2A and 2B, until they come locally intocontact (here via the tops of the protuberances 4) with the substrate 1(here, with the layer 6′), at a very precise temperature T_(c).

As shown in FIG. 7 b between the dash-dot lines, the protuberances 4limit the contact surface between the suspended part 2 of themicrobolometer and the substrate 1. This arrangement facilitates thereturn of the part 2 to its nominal position when the microbolometercools down, without which the surface adhesion forces could keep itpinned down to the substrate.

In order to obtain a firm contact of the protuberances 4 on thereflector 6, it is advantageous to choose a structure with highrigidity. This rigidity is also necessary in order to ensure a return totheir initial position of the deformed parts, after the part 2 has comelocally into contact with the substrate 1: the elastic energy stored inthe structure during the thermal expansion will be able to contribute tothe “detachment” of the surfaces in contact during cooling down. Asufficient number of support points will therefore be provided, andpreferably a coaxial configuration of the support arms.

On the other hand, the multiplication of the support points and supportarms is in principle accompanied by a reduction in the microbolometer'sown thermal resistance, and of its useful surface, and therefore itsperformance values.

A variant embodiment of the invention consists of producing, during astep preceding the steps described above, contact protuberances 5 on thesurface of the substrate 1. These protuberances 5, which have a smallsurface compared with the surface of the microbolometer, can ifappropriate be obtained by means of the natural topography of thereadout circuit, involving no procedure or precaution other than ajudicious positioning under the suspended part of the microbolometer. Ifthis variant is chosen, it may be unnecessary to provide protuberances 4on the microbolometer, and it is therefore possible to economize on thelithographic mask and the associated processes intended to produce theprotuberances 4 (i.e. to forgo etching of the layer 7 during the stepdescribed with reference to FIGS. 3 a and 3 b).

A second embodiment of the invention, represented in FIGS. 8 a and 8 b,consists of essentially implementing the same manufacturing steps as thefirst embodiment, except that the parts 2B are arranged “below” theparts 2A.

Here also, it is possible to provide protuberances 4 on the suspendedpart, or protuberances 5 on the substrate 1, or both types at the sametime. In FIGS. 8 a and 8 b, it has been chosen by way of example torepresent protuberances of the second type 5, which are situated facingthe median axis (dashed line and parallel to the X axis in FIG. 8 a) ofthe suspended part 2. FIG. 8 b shows between the dash-dot lines how thesuspended part 2 is deformed under the effect of heat until the zonesituated along this median axis comes locally into contact with theprotuberances 5.

This second embodiment makes it possible, by comparison with the first,to more easily arrange the contact zones 4 or 5 close to the centre ofthe microbolometer. In fact, the thermal deformation leads, in thisembodiment, to a concave surface 2, such that it is the zones situatedalong said median axis which drop furthest towards the substrate 1 whenthe temperature increases, contrary to the first embodiment in which therise in temperature leads to a convex surface 2.

It will be noted that the support arms 12 extend here in the direction Xperpendicular to the direction Y along which the parts 2B extend, inorder to effectively retain the suspended part 2, during its centraldrop, at the middle of each arm 12.

Of course, the nature of these deformations is reversed if the part 2Bis made from with materials with a lower thermal expansion coefficientthan those of the part 2A. A person skilled in the art naturally has thefreedom to define his materials, and also to adapt the relativepositions of the constituents, as a function of his practical needs,whilst remaining within the scope of the invention.

In particular, with the aim of simplifying the manufacture of radiationdetectors according to the invention, but at the cost of reducedreliability, it is possible to omit all the protuberances 4 and 5.

The effects of the invention are illustrated (with arbitrary scales) inFIG. 9, which is a graph of the temperature reached by a microbolometeras a function of the power P of the radiation received.

The mechanical contact at T=T_(c), of the suspended part 2 and thesubstrate 1, is followed by a drop in the temperature of the whole ofthe part 2, by thermal conduction, since T_(sub)<T_(c). The contactthermal resistance will be called R_(thc). R_(thc) depends on theeffective area of the contact, the nature of the materials in contact,the geometrical configuration (shape of the parts 2A and 2B, number andshape of any contact zones at the top of the protuberances 4 and 5), andthe nature of the materials constituting the parts 2.

It will be noted in this respect that, in the case where an embodimentis chosen including protuberances 4 and 5, the limitation of the contactsurface which results from this has the effect of reducing the rate ofcooling down of the suspended part 2 in contact with the substrate 1. Aperson skilled in the art must therefore seek the best compromisebetween ease of detachment and rate of cooling down.

Once the part 2 is in contact with the substrate, the overall thermalresistance, which will be designated R_(thg), is constituted by twoparallel resistances, i.e.:R _(thg) =R _(thc) ·R _(thb)/(R _(thc) +R _(thb))(it will be recalled that R_(thb) describes the microbolometer's “own”thermal resistance).

As a function of the choice of materials, design and construction of thevarious elements according to the invention, a person skilled in the artcan obtain thermal resistances R_(thb) of the order of some 10⁵ to 10⁶K/W, i.e. values which will easily be from ten to a hundred timessmaller than the microbolometer's own thermal resistance. The overallthermal resistance R_(thg) will therefore be of the order of the contactresistance R_(thc).

For moderate values of P, the temperature increases linearly, instandard manner. For received powers of the order of(T_(c)−T_(sub))/R_(thb), the microbolometer according to the inventionoscillates (mechanically and in temperature), due to the alternation ofphases of cooling in contact with the substrate and reheating after lossof contact.

For values greater than the received power, the part 2 is keptpermanently in contact with the substrate, at an average temperatureclose to (T_(sub)+P·R_(thg)).

The values of R_(thg) being hypothetically clearly smaller than R_(thb),accidental temperature deviations in the case of over-illumination aremoderate compared with the deviations (represented by the dottedstraight line in FIG. 9) which would be reached by a standardmicrobolometer.

Let T_(d) be the temperature starting from which the microbolometer mustbe considered to have deteriorated or been destroyed, and P_(d) thecorresponding power received. For a given value of T_(d), the inventionmakes it possible, by comparison with the prior art, to increase thepower which can be applied before destruction by a factor equal toP _(d2) /P _(d1) =R _(thb) /R _(thg),a factor of the order of ten or a hundred depending on themicrobolometer's design details.

In the above description, reference has been made to microbolometers inorder to fix ideas, but in fact the elementary detectors covered by theinvention can be of many kinds, and not only microbolometers; similarly,the range of wavelengths which they can detect is by no means limited tothe infrared, and the uses of these detectors cover many fields (forexample spectroscopy), and not only imaging. The invention can be used,in its various aspects, to benefit various devices which may suffer fromexcessive heating during use, such as for example MEMS(Micro-Electro-Mechanical Systems).

1. A microbolometer comprising a suspended part containing aradiation-sensitive element having a variable electrical resistancedepending on an intensity of radiation, wherein the suspended partcomprises a set of first zones and a set of second zones, the sets offirst and second zones being superimposed, and wherein the materialsconstituting the first zones possess thermal expansion coefficientssufficiently different from the materials constituting the second zonessuch that the suspended part is deformed under the effect of a rise intemperature to the point of coming into contact with an associatedsubstrate when a contact zone reaches a temperature T_(c) below thedestruction temperature T_(d) of the microbolometer.
 2. Themicrobolometer according to claim 1, wherein the first zones aresituated below the second zones.
 3. The microbolometer according toclaim 1, the first zones are situated above the second zones.
 4. Themicrobolometer according to claim 1 wherein the first zones possess athermal expansion coefficient smaller than that of the second zones. 5.The microbolometer according to claim 1, wherein the first zones possessa thermal expansion coefficient greater than that of the second zones.6. The microbolometer according to claim 1, wherein the suspended partincludes protuberances turned towards the substrate.
 7. Themicrobolometer according to claim 1, wherein the substrate includesprotuberances turned towards the suspended part.
 8. The microbolometeraccording to claim 1, wherein the suspended part includes support armsresting locally on the substrate by means of struts.
 9. Themicrobolometer according to claim 1, wherein the contact zone includesthe center of the suspended part.
 10. The microbolometer according toclaim 1, wherein contact zone is remote from the center of the suspendedpart.
 11. A radiation detector comprising an array of microbolometersaccording to claim
 1. 12. An observation or measuring apparatus,including at least one radiation detector according to claim
 11. 13. Amethod for manufacturing microbolometers on a substrate, comprising thefollowing steps: depositing and etching several layers, including a setof layers forming a suspended part of a microbolometer containing aradiation-sensitive element having a variable electrical resistancedepending on an intensity of radiation; and forming a set ofsuperimposed first zones and a set of second zones in the suspendedpart, wherein the materials constituting the first zones possess thermalexpansion coefficients sufficiently different from the materialsconstituting the second zones such that the suspended part is deformedunder the effect of a rise in temperature to the point of coming intocontact with an associated substrate when the contact zone reaches atemperature T_(c) below the destruction temperature T_(d) of themicrobolometer.
 14. The manufacturing method according to claim 13,further comprising producing protuberances emanating from the suspendedpart and turned towards the substrate.
 15. The manufacturing methodaccording to claim 13, further comprising producing protuberancesemanating from the substrate and turned towards the suspended part. 16.The manufacturing method according to claim 13, further comprisingproducing struts resting on the substrate and producing arms of thesuspended part resting locally on the struts.